One of the known optical communication systems transmits light signals through an optical fiber (see Patent Literatures 1 and 2 and Non-Patent Literatures 1-8). In an optical communication system, the received light contains a signal component and a noise component. A noise component contains components derived from self phase modulation, cross phase modulation, and spontaneously emitted light.
An example of an optical communication system changes the power of a signal to be transmitted and detects the power of a noise component contained in received light for each of different powers. The optical communication system estimates a ratio of the power of a signal component of the received light to the power of the component being derived from self phase modulation and being contained in the noise component of received light.    [Patent Literature 1] Japanese Laid-Open Patent Publication No. 2009-21943    [Patent Literature 2] Japanese Laid-Open Patent Publication No. 2013-229718